12 GEOLOGICAL MEMOIRS. 



he should have eliminated the type Gyroceras, the forms distin- 

 guished by that name being joined to those which have been called 

 Lituites. 



M. Barrande, while treating of the specific denominations which 

 he has adopted for the 850 to 900 new forms of Cephalopods dis- 

 covered in Bohemia, mentions the great difficulty he has found in 

 determining forms which he should consider specifically independent. 

 In almost all cases specific names have been preferred to the 

 names of varieties, which are always found inconvenient in practice, 

 especially when several exist in the same species. 



The general results of the geological and geographical distribution 

 of the Cephalopods can be sketched and more plainly indicated by 

 the particular distribution of each type in proportion to its richness 

 in specific forms. Thus a study of the genus Cyrtoceras^ of which 

 the author has nearly 460 forms, leads to certain conclusions and 

 considerations which, with but very little modification, could be 

 applied to the whole of the Palajozoic Cephalopods. 



One of the most interesting results of these researches is that these 

 moUusks present a very different order of development in the two 

 great Silurian zones of Bohemia — -the northern and the central. In 

 the primordial fauna no vestige of this order of mollusks has been 

 discovered. This horizon seems to be the starting-point from which 

 is manifested the difference in the relative level upon which appear 

 successively the diiferent types of Cephalopods, and in the relative 

 epoch of the greatest specific richness of each, as also in the maximum 

 of the total richness of the family of Nautilidae. The contrast is the 

 more remarkable because the generic types of this family, so unim- 

 portant in the number of their species, are represented as well in 

 the northern as in the central zone, and establish between their 

 faunas a connexion not otherwise met with, which appears to indi- 

 cate their relative contemporaneity. 



The author then contrasts the first appearance of the several genera 

 in the three Silurian faunas of the two zones, summing up by observ- 

 ing that in the northern zone the maximum richness is attained in 

 the second fauna, and is followed by a very marked diminution in 

 the third fauna ; while, on the contrary, in the central zone the mini- 

 mum is found in the second fauna, and the maximum is reached in 

 the third fauna in the most pronounced manner. M. Barrande con- 

 cludes by indorsing the opinion of MM. d'xirchiac and de Yerneuil, 

 that the genera which have the greatest horizontal diff'usion on the 

 globe, and are most often the richest in species, are also those 

 which have the greatest vertical range in the geological series. At 

 the same time this law has some remarkable exceptions. 



In a concluding chapter, M. Barrande points out the progress 

 which palaeontology has made since the publication of the memoir 

 by MM. d'Archiac and de Yerneuil on the fossils of the old deposits 

 of the Ehenish provinces*. By a series of comparisons of the species 

 from the Palseozoic strata, he shows that the entire class of mollusks 



- . * Trans. Geol. Soc, 2nd ser.vol. iv. p. 303. 



